The present invention relates to evaporators used in refrigeration chillers. More particularly, the present invention relates to an evaporator in which a pattern of flow in the liquid pool found in the evaporator shell is established and managed so as to accomplish and enhance lubricant return from that pool to a chiller system compressor.
Refrigeration chillers are machines which produce chilled water, most often for use in building comfort conditioning or industrial process applications. Such chillers typically employ a compressor to compress a refrigerant gas from a lower to a higher pressure. The higher pressure gas discharged from such a compressor is delivered to the chiller""s condenser where it is cooled and condenses to liquid form.
The refrigerant is then delivered from the condenser to and through an expansion device, which lowers the pressure of the refrigerant and still further cools it by the process of expansion. From the expansion device, the refrigerant is delivered to the system evaporator where it absorbs heat which is carried into the evaporator from the heat load which it is the purpose of the chiller to cool. As a result of the heat exchange process that occurs within the evaporator, the refrigerant vaporizes and is drawn back to the compressor where the process begins anew.
Because of the nature of compressors used in refrigeration chillers, a portion of the lubricant used within such compressors, which most often will be oil, makes its way into the stream of refrigerant gas that is discharged from the compressor. At least some of such lubricant is carried into the system condenser entrained in the stream of refrigerant gas that is discharged from the compressor. While various oil separators and oil separation schemes can be and are employed to remove the majority of the lubricant from the gas stream discharged from a compressor, at least a relatively small portion of such lubricant does make its way into the system condenser.
As hot refrigerant gas delivered into a chiller condenser condenses, it falls to the bottom thereof together with any lubricant that has been carried into the condenser or, in the case of an air-cooled condenser, the vapor is swept out of the condenser as a result of refrigerant flow. The condensed refrigerant and oil then flow, as noted above, from the condenser through an expansion device and into the chiller""s evaporator. If the lubricant that is carried into the chiller""s evaporator is not returned to the compressor from the evaporator on a continuous basis, it will accumulate in the evaporator and the compressor will eventually become starved for oil. Further, as lubricant concentration builds within an evaporator, the thermal performance of the evaporator comes to be more and more adversely affected.
Recently, both evaporator and chiller system design have undergone significant change, primarily in an effort to enhance overall chiller efficiency, but also to reduce the amount of refrigerant that is required to be used in chillers of a given capacity. Such changes are found in many aspects of chiller design. Two of the more prominent ones of such changes relate to the kind and nature of both the compressor and evaporator used in chiller systems, particularly in chillers generally in the 70-500 refrigeration ton capacity range.
In that regard, so-called flooded evaporators have historically been used in chiller systems in the 70-500 refrigeration ton capacity range as have been large capacity reciprocating or small capacity centrifugal chillers. In the late 1980""s and early 1990""s compressors of the screw type came to be developed and employed in chillers within that capacity range. While superior in many respects to large reciprocating and small centrifugal compressors in chillers within that capacity range, screw compressors, by their nature, cause a relatively large amount of oil to be entrained the stream of gas that is discharged from them. As a result, oil separation, management and return in chiller systems employing screw compressors is a more complex and critical undertaking.
In the mid-1990""s, evaporator technology evolved and resulted in the employment of so-called falling film technology in certain chillers generally in the 70-500 ton capacity range. The move to falling film evaporator designs was driven, in part, by the increasing expense of refrigerants used in refrigeration chillers. Falling film evaporators, by their nature, reduce the amount of refrigerant employed in chillers as compared to chillers of similar capacity which employ flooded evaporators.
In that regard, flooded evaporators require the use of larger refrigerant charges because the evaporator shell must contain enough liquid refrigerant to immerse the large majority or all of the tubes of the evaporator tube bundle. In falling film evaporators, on the other hand, liquid refrigerant is distributed and deposited in smaller amounts onto the tube bundle from above and generally across the length and width thereof. Such liquid refrigerant trickles downward through the bundle in the form of a film and only a relatively small percentage of the tubes of the tube bundle are immersed in a liquid refrigerant pool at the bottom of the evaporator shell. The result, once again, is to significantly reduce the size of the chiller""s refrigerant charge. In the case of both flooded and falling film evaporators, however, lubricant does make its way into the interior of the evaporator shell and into the liquid pool found therein.
Even though falling film evaporators have proven to be highly efficient and reduce the size of refrigerant charges used in chiller systems, their employment does bring with it associated costs and complexities that can offset the savings gained by reducing the size of a chiller""s refrigerant charge. This is particularly true in the lower portion of the 70-500 ton capacity range. Such complexities relate, among other things, to the process and apparatus by which oil is returned from a falling film evaporator to the system compressor and to the need, for the sake of efficiency, to achieve uniform distribution of liquid refrigerant across the length and width of tube bundles in such evaporators.
Because of certain of the complexities and the relative expense associated with the employment of falling film evaporators in refrigeration chiller systems, particularly those generally at the lower end of the 70-500 ton capacity range, and despite the advantages of the use thereof in terms of overall system efficiency and reduced refrigerant charge, the need continues to exist for still further advanced and/or differentiated evaporator designs which are of comparable or increased benefit and efficiency yet which are relatively less complex and/or expensive to employ.
It is an object of the present invention to provide an evaporator for a refrigeration chiller system that is economical of manufacture, efficient with respect to its thermal performance and the design and operation of which enhances the process of oil return to the system compressor.
It is a further object of the present invention to proactively establish a flow pattern in the pool of liquid refrigerant and oil that is found in refrigeration chiller evaporator and to proactively manage that flow so as to concentrate oil within that pool at a predictable location.
It is another object of the present invention to provide a chiller evaporator which by its operation delivers lubricant to a predictable location therewithin and in which thermal efficiency is enhanced by maintaining relatively very low oil concentrations at and around the large majority of the immersed tube surface within the evaporator shell.
It is still another object of the present invention to achieve high thermal performance and excellent lubricant management in the evaporator of a refrigeration chiller by managing liquid refrigerant flow within the evaporator shell so that a pattern of oil movement within the liquid pool at the bottom of the shell is established which delivers oil to a location from where it can easily be removed.
It is another object of the present invention to provide an evaporator for chiller systems of small to medium capacity which, by the application of certain features and concepts generally associated with falling film evaporators to what would otherwise be categorized as flooded evaporators, are made more cost effective overall than falling film evaporators, are generally equal thereto in terms of thermal performance and in which oil concentration is predictably managed to facilitate the return of such oil to the chiller""s compressor.
It is a further object of the present invention to provide an evaporator for chiller systems of medium to relatively larger capacity which, by the employment of managed flow in the liquid pool at the bottom of the evaporator shell and features primarily associated with falling film evaporators, together with apparatus for displacing liquid refrigerant generally to one end of the evaporator shell prior to its entry into the liquid pool, achieves effective lubricant management and return while maintaining and/or exceeding the thermal efficiency of current falling film evaporators.
These and other objects of the present invention, which will be apparent when the following Description of the Preferred Embodiment and attached Drawing Figures are considered, are achieved in a refrigeration system in which refrigerant is delivered into an evaporator shell above both the tube bundle and the liquid pool found therein and in which such refrigerant and any lubricant carried therein is deposited generally onto one end of the liquid pool from where its flow is managed so that lubricant concentrates in a predictable pool location. In that regard, vaporization of liquid refrigerant within that pool sets the pool in motion in a direction away from the location where liquid refrigerant and the lubricant carried therewith is deposited onto the pool surface. Because the liquid pool in the evaporator shell is placed in constant, managed motion in a direction from one end of the shell to the other, lubricant in that pool is caused to continuously flow to one predictable location within the pool in a manner which maintains oil concentration the majority of the liquid pool relatively very low. By maintaining lubricant concentration throughout the majority of the length of the liquid pool relatively very low and by causing lubricant to concentrate in a predetermined pool location from which it can relatively easily be removed, the thermal performance of the evaporator is maintained at a high level while oil return from the evaporator to the system compressor is both simplified and enhanced.